Input filter for image frequency suppression

Abstract

An input filter is provided for a superheterodyne receiver for image frequency suppression and a method for operating a superheterodyne receiver. The input filter for a superheterodyne receiver includes a first filter circuit with bandpass characteristics and a center frequency that can be preset as a function of a desired receive frequency, the circuit having a varactor diode and a first filter inductor that are connected in parallel and form a parallel-resonant circuit, wherein the center frequency can be preset by application of a control voltage to the varactor diode, and a second filter circuit with band stop characteristics that includes the varactor diode and a second filter inductor, these being connected in series and forming a series-resonant circuit, and that has a maximum attenuation in the vicinity of an image frequency belonging to the desired receive frequency. The input filter may be used in, for example, DAB receivers.

1 . An input filter for a superheterodyne receiver comprising:
 a first filter circuit having bandpass characteristics and a center frequency that is preset as a function of a desired receive frequency, the first filter circuit having a varactor diode and a first filter inductor that are connected in parallel and form a parallel-resonant circuit, wherein the center frequency is preset by application of a control voltage to the varactor diode; and   a second filter circuit having band stop characteristics that includes the varactor diode and a second filter inductor being connected in series and forming a series-resonant circuit, and that has a maximum attenuation in the vicinity of an image frequency belonging to the desired receive frequency.   
   
   
       2 . The input filter according to  claim 1 , wherein the second filter inductor is connected between a connecting node of a first terminal of the varactor diode and a first terminal of the first filter inductor and a ground potential. 
   
   
       3 . The input filter according to  claim 2 , further comprising a first DC decoupling capacitor, which is connected between the connecting node of the first terminal of the varactor diode and the first terminal of the first filter inductor and the second filter inductor. 
   
   
       4 . The input filter according to  claim 1 , further comprising a second DC decoupling capacitor, which is connected between a second terminal of the varactor diode and a second terminal of the first filter inductor. 
   
   
       5 . The input filter according to  claim 1 , further comprising a reference voltage connection resistor, which is connected between the second terminal of the varactor diode and a ground potential. 
   
   
       6 . The input filter according to  claim 1 , further comprising a third DC decoupling capacitor, which is connected between a second terminal of the varactor diode and an input terminal of the input filter. 
   
   
       7 . The input filter according to  claim 1 , further comprising a control voltage connection resistor, which is connected between a connecting node of a first terminal of the varactor diode and a first terminal of the first filter inductor and a control voltage source. 
   
   
       8 . The input filter according to  claim 1 , further comprising a fourth DC decoupling capacitor, which is connected between a second terminal of the first filter inductor and an output terminal of the input filter. 
   
   
       9 . A method for operating a superheterodyne receiver, in which an input signal of the superheterodyne receiver is filtered with a bandpass characteristic, the method comprising:
 presetting a center frequency of the bandpass characteristic as a function of a desired receive frequency; and   filtering the input signal with a band stop characteristic whose maximum attenuation lies in a vicinity of an image frequency belonging to the desired receive frequency.